Klaus Mikaelson might also be considered the most powerful. His hybrid status gives him super strength, speed, and the ability to compel others. In the world of Game of Thrones, he could easily use his compulsion on many of the characters to get what he wants. He's also very difficult to kill, which would give him an edge over most characters from Game of Thrones.
If we look at the character of Nico di Angelo from Percy Jackson, his ability to control the dead could be a game - changer when dealing with the vampires in The Originals. Vampires are technically undead, so Nico might be able to manipulate them to some extent. However, the witches in The Originals have their own magic to resist such things. And on the other side, characters like Elijah Mikaelson are very skilled fighters and diplomats. Their charm and combat skills could make them a formidable opponent against the Percy Jackson characters. In the end, it's all about the creativity of the fanfiction writer in determining who is the most powerful in these crossovers.
Arya Stark and Rebekah Mikaelson. Arya is a trained assassin, and Rebekah is a fierce vampire. They could learn a lot from each other. Arya could teach Rebekah some of her stealth and combat skills without the use of vampire powers, and Rebekah could introduce Arya to the world of the supernatural and its different rules of survival.
It could be Jon Snow. If he has a Stand in the fanfiction, combined with his already proven leadership skills and combat abilities, he would be a force to be reckoned with. His ability to come back from the dead might also be related to his Stand powers in some way, making him even more formidable.
Eragon would likely be one of the most powerful. His bond with Saphira gives him access to powerful dragon - based magic. He can also use the ancient language to perform various spells. In the power - hungry world of Game of Thrones, his abilities would make him a force to be reckoned with.
Sauron, if he were in his full power, would be terrifying. His ability to control minds and his dark magic could potentially subdue many of the 'Game of Thrones' characters. But he might underestimate the political cunning and the will of some of the more tenacious Westeros leaders, which could lead to his downfall in this fictional battle.
It's likely that Percy Jackson himself would be very powerful. His ability to control water gives him a huge advantage, especially in a world like Westeros where there are many bodies of water. He could drown his enemies or use water to deflect attacks. However, Daenerys with her dragons would also be a formidable opponent. Her dragons could breathe fire and fly, which is hard to combat.
It's likely Klaus Mikaelson. He's an Original vampire - werewolf hybrid in The Originals. He has super strength, speed, and his werewolf side gives him an edge in combat. In comparison, the vampires in Twilight, while strong, don't have the added advantage of being part werewolf. Also, Klaus has a lot of experience and is very cunning, which would make him a formidable force in a crossover situation.
It might be Ruby. Her speed and her scythe skills could make her a formidable opponent in the battlefields of Westeros. She could quickly move around and take out enemies before they even realize what's happening.
Well, it could also be argued that Daenerys Targaryen might be very powerful in such a fanfic. She has her dragons, which are extremely powerful weapons. If she were to enter Narnia, she could potentially use her dragons to gain a stronghold. However, she would still be no match for Aslan in terms of pure power and divine essence.
Perhaps Daenerys Targaryen. If she were to somehow acquire vampire - like abilities, she would be an even more formidable force. Her dragons combined with vampire powers such as enhanced senses and immortality (in a sense) would make her a very powerful and dangerous character. She could use her newfound powers to rule Westeros more effectively or to take on new enemies.